I had my first significant experience with a 5e "solo" (legendary monster) on Sunday. We were 12th-level. I would recommend stealing that type of monster design, then flavoring abilities like PC abilities. (So a legendary wizard might still dish out Magic Missile, but it's an epic missile storm that can deliver far more damage than a PC wizard could against multiple opponents. And then the epic wizard has other actions!)
The solo wasn't the only guy on the battlefield, which was very large; opponents attacked us from three directions (three of which were significant enough in their own right that I didn't think of them as "minion") and the solo hadn't even appeared on the field at the start. One of our PCs (an elf ranger) was taking damage from being shot at with arrows. My character (a dwarven fighter) had only taken 3 damage taking on some minor opponents, but unfortunately this took me further away from the solo's intended attack point. Then the solo appeared out of the gloom and teleported to the critical point and still had actions left over since they could use legendary actions.
One PC dimension doored with my dwarven fighter to that point, but by then the ranger had taken a full round of attacks to the face. Some of the attacks weren't even directed at him, but he was hit at least four times that round. Together with the damage he had already taken we found ourselves looking up how far into negative hit points can a character fall without dying. (Apparently more than 4e; minus your total hit points!) So he didn't die outright, but was currently lying on the ground, face up, with a startled expression on his reshaped face.
My character had 103 hit points left when he got there, and took four attacks, reducing him to exactly 1 hit point! Second Wind would not give him enough hit points, and Parry would not cut enough incoming damage, to save him without some urgent assistance. In the end, we were all mangled. I would have been happier if we had interacted with the legendary monster earlier, but we fought a battle that could have easily ended up as a TPK. When you find yourself frantically looking over your character sheet, trying to find options that will keep you alive, you know you've bitten off more than you can chew.